\name{snellen}
\alias{snellen}
%- Also NEED an '\alias' for EACH other topic documented here.
\title{Convert Snellen Values}
\description{Calculate logMARs from Snellen Values (character data).  This is an internal function and is not typically meant to be called directly.}
\usage{
snellen(snellenvalue, chart.values, chart.nletters)
}
\arguments{
  \item{snellenvalue}{
%%     ~~Describe \code{snellenvalue} here~~
}
  \item{chart.values}{
%%     ~~Describe \code{chart.values} here~~
}
  \item{chart.nletters}{
%%     ~~Describe \code{chart.nletters} here~~
}
}
\details{
  This is basically just a stub for now.  I copied some of the comments from my code over, but it needs to be expanded and cleaned up.

  The input should be character data and have a numerator and denominator separated by '/'.  E.g., "20/20", "20/40 + 3".  This handles both simple and Snellen values that need to be interpolated given the appropriate chart.

  logMAR calculations including interpolating partial lines.  Given "20/25 + 3", calculate the logMAR of 20/25 and 20/20 (the next step up since '+'), and go 3/chart.nletters of the way between these two values.  Similarly if "20/20 - 3", it will go partway between 20/25 and 20/30.  Note that it depends on the lines and letters per line \emph{on the actual chart used}. The \code{chart.values} and \code{chart.nletters} should contain all the lines and number of letters for the chart that was used.
}
\value{Stub.
}
\references{Jack T. Holladay (2004).  Visual acuity measurements.  \emph{Journal of Cataract & Refractive Surgery, 30}(2), pp. 287--290. DOI: 10.1016/j.jcrs.2004.01.014.  \url{http://www.sciencedirect.com/science/article/B6VSF-4BX5RWS-2/2/c76a1a79f122eed8ace7be814131f749}}
\author{Joshua Wiley, \url{http://joshuawiley.com/}}
\seealso{\code{\link{logmar}} for the function that does the final conversion, \code{\link{VAConverter}} the overall function typically called, \code{\link{CFHM}} to parse CF or HM data to pass to \code{logmar}.
}
\examples{
## None.
}
\keyword{misc}
